This group includes alloys where particularly Cr and Mo are added in modest amounts to improve the high temperature strengths and creep resistance. Temperature applications run up to approximately 1100F. The alloys are also produced to maintain weldability characteristics.
These alloys must be heat treated to achieve their full property potential. The heat treatments include a normalize and temper treatment. The objective is to obtain a stable micro structure. Long and closely controlled temper heat treatments are used at times to assure the elimination of residual stresses.
Similar to carbon steels and low alloy steels, melting of these alloys require attention to the problem that they are gas pickup prone. They will pick up H from anything wet in the charge and they will pick up N from the atmosphere if held liquid in the furnace. Their melting is further complicated by the absorption of O from the atmosphere and requires exacting additions of Al to offset this tendency. Castability of this group of alloys is somewhat impaired because of poor fluidity. Also similar to low alloy steels, these alloys require precautions to avoid cracking. Thermal processing in the finishing department requires pre-heat of the parts. The pre-heat minimum temperatures are usually contained in the material specification. It is also common practice to final inspect castings in this group beyond visual inspection with non-destructive techniques to assure a crack free part.
Applications for materials in this group are centered in power generation and transmission fields. Valve bodies, turbine parts, etc. handling superheated steam are good examples.